Intercorrelation among surface chemical composition, packing structure of molecules, water contact angles, amounts and structures of adsorbed proteins, and blood compatibility was systematically investigated with self-assembled monolayers (SAMs) with continuous chemical composition gradients. The SAMs were mixtures of two thiols: n=hexanethiol (hydrophobic and protein-adsorbing) and hydroxyl-tri(ethylene glycol)-terminated alkanethiol (hydrophilic and protein-resistant) with continuously changing mixing ratios. From the systematic analyses, we found that protein adsorption is governed both by sizes of proteins and hydrophobic domains of the substrate, Furthermore, we found a clear correlation between adsorption of fibrinogen and adhesion of platelets. Combined with the results of surface force measurements, we found that the interfacial behavior of water molecules is profoundly correlated with protein resistance and antiplatelet adhesion. On the basis of these results, we conclude that the structuring of water at the SAM-water interface is a critical factor in this context.